Once the points are generated, the tessellator launches one thread per point to the TES.įinally, the TES evaluates and transforms points into clip space to de. While the amount of tessellation is controlled by data output by the TCS, the TES declares an input which controls the pattern of triangles generated. All it really does is generate a sequence of u.v coordinates and an associated topology map to control how the patch is converted to triangles. Moving on the tessellator, it is very much a black-box from the point of view of the graphics programmer. (These are more advanced features that we’ll skip in this sample) One thing that makes the TCS special, is that the threads can all see all of the input data, and at the end of the shader, they can share their results to allow group computations for items like patch-wide tessellation levels. layout (vertices=1) out ) In the TCS, each output will have its own thread to generate the output vertices. The TCS specifies the number of vertices it creates in a patch via a layout statement. The TES takes the parametric coordinate and the patch data output by the TCS to generate a final position for the surface.ĭiving in a bit deeper, the TCS is one of the more complex shader stages in the pipeline due to all of the options it controls. (For our example today, the space is a quadrilateral with u and v directions.) Finally, the “tessellation evaluation shader” (TES) is executed at least once for each vertex that was created in the parametric space. The tessellator takes the amount of tessellation provided by the TCS and computes a pattern of triangles in a parametric space. Otherwise just the glClipDistance built-in needs to be redeclared. With GLSL 4.10 or ARBseparateshaderobjects, the whole glPerVertex block needs to be redeclared. In order to use this variable, the user must manually redeclare it with an explicit size. The next tessellation stage is the fixed-function tessellator. Each element in the array is one clip plane. This shader accepts a list of vertices defining a patch from the vertex shader and controls the amount of tessellation applied to the patch. The initial stage is the known as the “tessellation control shader” (TCS). These new tessellation stages operate between the vertex and geometry shader stages previously available in the pipeline. It all fits into the pipeline like the diagram below. It expands the pipeline by two additional programmable stages and a single fixed-function stage that handle the specification and evaluation of a tessellated surface. OpenGL recently added the concept of tessellation shading to its definition of the graphics pipeline. The sample only demonstrates tessellating the quad domain (rather than the triangular domain), and it shows two modes: one with a flat plane and one where the plane is wrapped into a torus. As the name implies, it demonstrates the minimum parts necessary to use tessellation shaders in OpenGL. Return UnityEdgeLengthBasedTess (v0.vertex, v1.vertex, v2.vertex, _EdgeLength) įloat d = tex2Dlod(_DispTex, float4(v.texcoord.xy,0,0)).In the last OpenGL SDK post I introduced the new SDK, now let me introduce our first new sample, “Simple Tessellation Shader”. #pragma surface surf BlinnPhong addshadow fullforwardshadows vertex:disp tessellate:tessEdge nolightmapįloat4 tessEdge (appdata v0, appdata v1, appdata v2) It just moves vertices along their normals based on the amount coming from a displacement map: Shader "Tessellation Sample" This next example shows a surface shader that does some displacement mapping without using tessellation. No GPU tessellation, displacement in the vertex modifier When you use tessellation, the shader is automatically compiled into the Shader Model 4.6 target, which prevents support for running on older graphics targets.Only triangle domain - no quads, no isoline tessellation.Surface shaders can optionally compute phong tessellation to smooth model surface even without any displacement mapping.Ĭurrent limitations of tessellation support:.A geometry shader is optional and does not have to be used. Here you’d typically to displacement mapping. A Geometry Shader (GS) is a Shader program written in GLSL that governs the processing of Primitives.Geometry shaders reside between the Vertex Shaders (or the optional Tessellation stage) and the fixed-function Vertex Post-Processing stage. When tessellation is used, “vertex modifier” ( vertex:FunctionName) is invoked after tessellation, for each generated vertex in the domain shader.That function computes triangle edge and inside tessellation factors. Tessellation is indicated by tessellate:FunctionName modifier. Surface Shaders have some support for DirectX 11 / OpenGL Core GPU Tessellation.
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